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Friday, 10 March 2017

New rule to help PBS vehicles maintain safe speeds on descents

Austroads has published a report that proposes a new rule in the Performance Based Standards scheme, adding a requirement for applicant vehicles to meet a minimum level of auxiliary braking performance. The purpose of the new rule is to ensure that PBS vehicles can maintain a safe speed on long, steep descents, without reliance on service brakes.

Auxiliary brakes are speed control devices that are able to slow a vehicle without using its service brakes. With the correct use of a suitable auxiliary brake on a long downhill run, it should be possible to minimise the use of a vehicle’s service brakes and thereby maintain their capacity to stop the vehicle.

The principles and operational mechanisms of service brakes and auxiliary brakes are discussed as background information to the creation of the rule. Performance data on auxiliary brakes are used to calculate the theoretical capacity of such devices to aid in speed control during descents.

The project determined that vehicles presented for inclusion in the PBS scheme are generally available with sufficient auxiliary braking capacity to safely manage their speed on extended descents.

The proposed PBS rule aims to ensure that PBS vehicles are equipped with sufficient auxiliary brake capacity to:

  • maintain speed without accelerating on a 7% downhill grade
  • achieve certain minimum speeds on a 7% downhill grade, dependent on the requested level of PBS network access.

Example vehicle assessment calculations are included as a demonstration of the application of the proposed rule, using vehicle performance data obtainable from vehicle manufacturers. The text of the rule is ready for consideration as an addition to the PBS Standards and Vehicle Assessment Rules.

The report also examines the existing PBS rule on directional stability under braking. This review presents for consideration a number of changes to the deemed‑to‑comply aspect of the rule. These changes are designed to improve the standard of brake equipment fitted to PBS vehicles, but care in their application will be required so as not to inadvertently conflict with impending changes to Australian Design Rules.

Report Link: Developing Braking Standards for Heavy Vehicles to Brake Effectively and Safely on Steep Declines

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